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A simple model is presented for interpreting the presence of substantial second harmonic generation (SHG) activity from assemblies of centrosymmetric molecular building blocks. Using butadiene as a computationally tractable centrosymmetric model system, time-dependent Hartree-Fock calculations of the nonlinear polarizability of butadiene dimer were well-described through exciton coupling arguments based on the electronic structure of the monomer and the relative orientation between the monomers within the dimer. Experimental studies of the centrosymmetric molecule 2,6-di-tert-butylanthraquinone suggest the formation of a combination of SHG-active and SHG-inactive crystal forms. The structure for the centrosymmetric form is known, serving as a negative control for the model, while the presence of an additional SHG-active metastable form is consistent with predictions of the model for alternative molecular packing configurations.

During 1983 and 1984, ileocecal lymph node specimens were obtained from clinically normal cattle at 76 US Department of Agriculture-inspected cull cattle slaughterhouses in 32 states and Puerto Rico. Mycobacterium paratuberculosis was isolated from the lymph nodes of 119 of 7,540 cattle. The prevalence of bovine paratuberculosis was 1.6% overall, with 2.9% in dairy culls and 0.8% in beef culls. The prevalence for females and males did not appear to differ significantly.